Optical disk data storage systems enable the storage of great quantities of data on a disk. The data is accessed by focusing a laser beam onto the data layer of the disk and then detecting the reflected light beam. The data is represented by changes in reflectivity. Various kinds of systems are known. In a ROM (Read Only Memory) system, such as a compact disk system (CD-ROM), data is permanently embedded as marks in the disk at the time of manufacture of the disk. The data is detected as a change in reflectivity as the laser beam passes over the data marks. A WORM (Write-Once Read-Many) system allows the user to write data by making marks, such as pits, on a blank optical disk surface. Once the data is recorded onto: the WORM disk it cannot be erased. The data in a WORM system is also detected as a change in reflectivity. There are also erasable optical data storage systems, such as phase change and magneto-optic (M-O) systems. While phase change systems also read data by sensing a change in reflectivity, M-O systems read data by measuring the rotation of the incident polarization caused by the M-O media.
To increase the storage capacity of an optical disk, multiple data layer systems have been proposed. An optical disk having two or more data layers may be accessed at different layers by changing the focal position of the lens. IBM's U.S. Pat. No. 5,202,875 describes a multiple dam layer system wherein the optical medium comprises either a plurality of substrates spaced apart by air gaps or a plurality of laminated substrates, each of the substrates having a data layer. U.S. Pat. No. 4,450,553 assigned to U.S. Philips, uses laminated disk substrates that are bonded together without an air gap, with each substrate supporting a CD-ROM type data layer. In such systems, the focused light spot from the laser source must be able to be positioned from one data layer to any other data layer to read and/or write data to the different data layers.
For a conventional single data layer optical disk drive, the focus acquisition time for the optical spot on the data layer is achieved from a stationary starting position of the focusing lens while the disk moves up and down at some relative velocity due to its vertical or axial runout. In this case, the focus acquisition time does not have to be fast since it is done only once at disk drive startup. This time is typically about 1 second for conventional CD-ROM and high-performance optical disk drives. However, for multiple data layer disks, this long focus acquisition time when switching between data layers is not acceptable because it results in excessively long data access times for data on the different data layers. The time to acquire focus when switching from one data layer to another should be as small as possible and optimally significantly less than the data access time within a given data layer. The data access times for CD-ROM drives is about 200 ms and for high-performance optical disk drives about 30 ms. The switching between data layers in a multiple layer disk drive is complicated by the fact that the data layers are not perfect planes that remain perfectly parallel with one another. Instead they generally have considerable axial runout and are not necessarily separated by a fixed spacing distance. In particular with respect to the multiple data layer disk where the individual substrates are separated by air gaps, the actual spacing between two separate data layers varies as a function of radial and angular position on the disk. These factors result in a large relative axial velocity between the different data layers, which varies as a function of the disk radial and angular positions.
What is needed is a multiple data layer optical disk drive that has a reliable and accurate system for positioning the focused laser spot from one data layer to another to rapidly access the data on the different data layers.